Not some version of ‘grassing up’ or talking too much or talking out of turn….
It is the acronym several sports use to frame or draw attention to what specific key body parts of the athlete are doing or should be doing, most appropriate to the Sport.
Each element is described and explained, and when they are all put together they give us a picture of what is happening and what we should be looking for.
Just take the first letter of each element and for SWIMMING we have….:

B Body Its’ position in the water; its’ ‘profile’ and characteristics if you like.
L Legs Their movements and actions as well as ‘profile’ and characteristics
A Arms Their movements and actions as well as profile and characteristics
B Breathing Specifically, the type and nature of being able to keep breathing
T Timing Pretty much all of the above when coordinated and mastered in a stroke or skill that somehow is beginning to look ‘easy’ and well practiced

A useful way to look at BLABT is in terms of the function or part each plays in Aquatics:

B Body Provide a stable ‘platform’ for movements of the legs and arms, while floating at or near the surface – most effective when horizontal.
L Legs Aquatic kicking’… is different. Legs tend to act either, alternately* or together** Almost instinctively trying to maintain or support the current body position.
For swimming their action is mostly to keep the body horizontal and balanced. They can provide some propulsion notably in Breaststroke.
A Arms Used mostly to provide the largest propulsive forces in strokes using the hands like ‘paddles. In most activities it is a sculling type of action.
Crucially the PROPULSIVE bits take place under the water. It consists of pressing, ‘pulling’ sweeping or sculling so that the swimmer is thrust or ‘propelled’ in one direction. If the swimmer wants to travel forward then that direction will be backwards, from front to back. In order to keep propulsion as continuous as possible, the hand and arm will need to RECOVER to the front again to repeat the powerful and accelerating action which will thrust the swimmer forwards through the water.
B Breathing ‘Aquatic’ breathing is most effective when it is controlled and rhythmic; working in ‘sync’ with everything else that is happening with ‘B’ – ‘L’ and ‘A’…..If we get it right, then we can keep doing whatever it is we are trying to do ! (See ‘Trickle’ and ‘Explosive’)
T Timing Think: “co-ordinated” and ‘effortless ease’.
When applied to an activity or a stroke we are talking about something that looks efficient and competent; relaxed and confident.
It’s the smooth, ‘fluent’ coordination and transition of all the above; it comes from repetition, guided practice and of course high-quality feedback.